As implanted in game not being a particularly concerning issue to me I personally define the core system like this:
The primary platform of the Mojlnir-powered assault armor.
This means: The undersuit, the powerplant, and the “Default” elements that largely remain unchanged by any level of customization.
From an art style perspective, these elements are typically: The torso, the codpiece, thighs, boots, inner shoulder mounts, and forearms.
with the above in mind, there are a few elements that I have always preferred in my personal taste when it comes to the various spartan armors.
* I have always preferred the “boxy” design of the vents for the fusion core, exemplified in the MK VI, MK VII, and MK VI Gen 3.
This is in my opinion one of the definitive design elements that make Spartan armor so recognizable.
* I have always liked elements that really hammer home the “walking tank” feel of the various armors. which is why I think I have always liked the MK IV so much for so long.
* Cohesive and purpose-built. form fitting and no floating parts.
As to dislikes.
There are things that I dislike, but they largely factor as direct counters to things that I like.
*Flowing lines for “art” sake, most recognizable in Halo 5, there are several armors that seem to levitate above the shoulders of the person wearing it rather than feeling like nearly a ton of titanium.
*Similar to SpartanSweed above, I have difficulty with armors that lack that definitive profile. MK Vb, SPI, and Rakshasa. but that is purely because I love the other profile so much.
If I had to make my own favorite I would have to say this: (I found it on Pinterest but I call it the MK IV Gen 3) and it is what I am working towards for my own armor. of course just in blue rather than green.
